Moving the 3D cursor while in Blender 2.8's Grease Pencil Draw Mode
It appears that this possibility is currently missing.

You can work around it by setting a key binding for your purpose. I suggest CtrlShiftLMB.


Open Edit > User Preferences > Input
Search "Set 3D cursor"
Edit the first entry (under "3D view"), by clicking on the current key binding and by "recording" CtrlShiftLMB (you can change it later to CtrlShiftAction Mouse for compatibility with left-click-select mode)


"Save User Settings"


It should work fine. Using LMB alone will still work in modes other than Draw, and the Lasso select should still work with CtrlShiftLMB if you do click-and-drag instead of click-and-release.